1. Field of the Invention
This invention relates to nuclear well logging method for determining the nature of fluids in formations through which a borehole is formed as well as the nature of fluids in the borehole. More particularly, the invention relates to determining the hydrocarbon saturation (or its correlative, water saturation) of formations adjacent a borehole by nuclear radiation logging. Still more particularly, the invention relates to inelastic gamma ray spectrum logging of a formation with correction for gamma rays from borehole fluids.
2. Related Art
A major goal of well logging is to establish the fraction of pore space in the earth formation occupied by hydrocarbons. Three methods of doing so have been developed in the prior art. The first two methods are electrical resistivity and thermal neutron decay methods which measure the water saturation S.sub.w, and the difference, S.sub.o =1-S.sub.w, is the saturation of all other liquids and gases. The term S.sub.o, or "oil" saturation, will be used hereafter to refer not only to liquid hydrocarbons, but also to gas. Both the electrical resistivity and thermal neutron decay methods depend upon the presence of salts dissolved in the water and, for that reason, are less effective in fresh water than in salt water environments.
The third known method is based on the fact that hydrocarbons contain carbon and water contains oxygen. When a high energy neutron, usually called "fast neutron", is scattered inelastically from a carbon atom, a 4.4 MeV gamma ray is emitted. When a fast neutron is scattered from an oxygen atom, a 6.1 MeV gamma ray is emitted among others. Therefore, a logging apparatus which counts the number of 4.4 MeV gamma rays and the number of 6.1 MeV gamma rays and determines their ratio should be, under ideal conditions, able to provide a measure of the ratio of carbon to oxygen in the formation. Such measurements are known in the art as carbon/oxygen or simply, C/O measurements or C/O logs. Moreover, a calcium/silicon ratio can also be obtained. Comparison of these two ratios permits the user to distinguish carbon in calcium carbonates from that in hydrocarbons.
In cased hole wells, where the salinity or salt content of a water-saturated zone is not known, is very low, or has been altered by production by water injection, the C/O measurement is the only alternative to resistivity and neutron decay methods.
Under actual field conditions, however, the well bore may contain hydrocarbons (in the form of oil or gas) and water. Consequently, C/O measurements of the formation are contaminated or "corrupted" with gamma rays resulting from fast neutron interaction with carbon and oxygen atoms of fluids in the borehole. In addition, lithologies, such as dolomite and limestone, contain carbon atoms. Such contamination of the inelastic gamma ray spectral data, and ultimately of the S.sub.o determination, may be eliminated if the porosity, lithology, borehole configuration and hydrocarbon content of the fluid in the well bore is known.
Characteristics as a function of depth of a cased well, such as porosity and lithology of the formation and the borehole configuration may be known. But C/O logging has been highly sensitive to uncertainty of the borehole oil/water mixture. For this reason C/O logging measurements of cased, producing wells have required that the well be "shut-in" so that the borehole fluid components may be known better. However, even with shut-in wells, the content of borehole fluid is not always known well enough.
As described in U.S. pending application Ser. No. 07/203,397 filed on June 7, 1988 in the name of McKeon, Roscoe, Stoller, and assigned to Schlumberger Technology Corporation, the C/O measurements are carried out by using a logging tool provided with a near and a far detector. The relative amounts of carbon and oxygen C.sup.n, O.sup.n as measured from the near detector and the relative amounts of carbon and oxygen C.sup.f, O.sup.f, as measured from the far detector, are obtained. At least squares analysis is performed to determine C.sup.n, O.sup.n from the energy spectrum (counts versus energy) acquired from the near detector, using standard spectra for the near detector, C.sup.f and O.sup.f are determined from the energy spectrum as measured from the far detector using standard spectra for the far detector. The analysis is performed at successive logging depth in the borehole. Next, the carbon and oxygen determinations of the near and far detectors are combined to determine oil saturation of the formation (S.sub.o) and/or the oil percentage in the borehole (C.sub.b). This is done by assuming that the total carbon and oxygen measured as indicated above are equal to the sum of the carbon and oxygen yields from the rock matrix of the formation, the pore space fluid, and the borehole fluid: EQU C=C.sub.mat +C.sub.por +C.sub.bh ( 1) EQU O=O.sub.mat +O.sub.por +O.sub.bh ( 2)
where "C" and "O" are the carbon and oxygen yields (as measured) and the subscripts stand respectively for "matrix", "pore space", and "borehole". The term "yield" refers here to the number of gamma ray counts coming from a specific element. As known from the US pending patent application referred to, equations (1) and (2) may be expressed as a function of S.sub.o (oil saturation in the formation, or percentage of oil in the pore space) and C.sub.b (the percentage of oil in the borehole): EQU C.sub.meas =.alpha.+.beta.S.sub.o +.delta.C.sub.b ( 3) EQU O.sub.meas =.eta.+.mu.S.sub.o +.nu.C.sub.b ( 4)
The coefficients .alpha., .beta., .delta., .eta., .mu. and .nu. are then determined under laboratory conditions by taking four measurements under the same conditions except varying S.sub.o and C.sub.b. For example, the conditions of a 10 inch borehole, a 7"(17.8 cm)-23 lb per foot (15.5 kg/m) casing in a 33 p.u. (porosity unit) sandstone formation may be established, for calibration, and then C and O from near and far detectors may be measured with the logging tool to be used in the field. The table below illustrates the laboratory measurements:
______________________________________ BOREHOLE FORMATION MEASURE ______________________________________ CONDITION water water C.sup.n, O.sup.n C.sup.f, O.sup.f water oil C.sup.n, O.sup.n C.sup.f, O.sup.f oil water C.sup.n, O.sup.n C.sup.f, O.sup.f oil oil C.sup.n, O.sup.n C.sup.f, O.sup.f ______________________________________
The superscripts "n" and "f" refer respectively to the near and far detector.
These four measurements with three unknowns are for near and far carbon and oxygen. Since the equations (3) and (4) are over determined, the coefficients .alpha., .beta., .delta., .eta., .mu., and .nu., for both the near and far measurements, are obtained using conventional least squares procedures.
Next, a carbon/oxygen ratio is formed for each of the near and far detectors, i.e. C.sup.n /O.sup.n and C.sup.f /O.sup.f, leading to two equations which are solved for S.sub.o and C.sub.b.
At each depth in the borehole, a signal representative of oil saturation S.sub.o, and water saturation S.sub.w =1-S.sub.o, and percentage oil in the borehole C.sub.b, is recorded.
Although the above referred known method represents a substantial improvement one could increase the benefit from the use of two different detectors by more efficiently combining or correlating measurements from respective detectors. Furthermore, the determination of the .alpha., .beta., .delta., .eta., .mu. and .nu. coefficients could be improved.
Consequently, while the known technology above described represents efforts to advance the nuclear logging art, the need remains for a method and apparatus by which the oil saturation in the formation (S.sub.o) and/or in the borehole fluid (C.sub.b) may be more accurately determined through C/O logging techniques with correction for corrupting gamma rays of unknown amounts of hydrocarbons in the borehole.